Not applicable.
The present invention relates generally to subsea pig launching, and more particularly to a method and apparatus for storing and sequentially launching multiple pigs from a remote location. Still more particularly, the present invention is a method and apparatus for launching a series of pigs over a relatively long period of time.
In the oil and gas industry, it is common to remove petroleum deposits and other debris from pipelines by pushing a cylindrical scraper, referred to as a xe2x80x9cpig,xe2x80x9d through the lines using fluid pressure. The pig is introduced into the pipeline from a launching system that typically includes a releasable retainer for retaining the unlaunched pig and a source of fluid pressure connected to the housing behind the pig.
Subsea satellite wells, manifolds or templates in deepwater are typically connected to a host platform located in shallow water via subsea flowlines that transport the produced hydrocarbon fluid along the sea floor. Such systems are often referred to as xe2x80x9ctie-backxe2x80x9d systems. As with other types of pipelines, flowlines in tie-back systems need to be pigged periodically during their operation to remove paraffin deposits, displace liquids, etc. The traditional method of pigging such flowlines has been to use xe2x80x9cround trip pigging,xe2x80x9d which requires a pair of parallel flowlines between the host platform and the manifold or the wellhead. Pigs are typically launched from and received at the host platform, traveling outward through one flowline and returning through the other.
A more economical option for deepwater flowlines is to use xe2x80x9csingle trip piggingxe2x80x9d using a single flowline between the manifold and the host platform. In single trip pigging, the pigs are launched from a pig launcher mounted on the subsea manifold and received on the host platform. The pig launcher can also be mounted on a wellhead or a pipeline end manifold (PLEM) sled, which is connected to production manifold/wellhead via jumpers.
In order to maximize the advantage of a single trip pigging system, the cost of offshore intervention in the system should be minimized. Such intervention is required either to replace the pigs in the launcher or to retrieve the launcher and recharge it with a new set of pigs after the previous set has been launched into the flowline. This implies that the pig launcher should be able to hold a large number of pigs and, for deepwater application, all pigging operations should be performed using an ROV or remotely from the host platform.
Furthermore, depending on the pigging operation frequency, the pigs are likely to be left in the launcher over a long period, as much as one to two years. However, since conventional pigs tend to degrade when exposed to hydrocarbons or methanol for a long period, conventional pigs cannot be left exposed to hydrocarbons or methanol while they are stored in the pig launcher. Hence, it is further desired to provide a pig launching system that does not store the pigs in either production fluids or methanol.
The present invention provides an apparatus for launching pigs into a subsea flowline that connects a subsea manifold, template or a wellhead to a hostplatform and provides a reloading apparatus that can be used in conjunction with an ROV and can carry several pigs and deploy them remotely into a subsea pig launching apparatus.
One preferred embodiment provides a piston pig for pushing pigs from a larger diameter conduit into a smaller diameter conduit. The piston pig includes a cylindrical body with a first sealing disc attached to the body and adapted to seal against the larger diameter conduit and maintain a pressure differential across the body of the piston pig. The piston pig also includes a second sealing disc attached to the body and capable of sealing against the smaller diameter conduit and maintain a pressure differential across the body. Certain embodiments also include a pressure control device, such as a resilient flapper disc, to limit the differential pressure across the body in a direction opposite to the intended direction of movement and allow fluid flow against the direction of the pig. Some piston pig embodiments may also include an end cap attached to, and slidable relative to, the body, where the end cap has a closed position in which fluid cannot bypass the end cap and an open position in which fluid can bypass the end cap so as to limit the differential pressure across the pig, thus limiting the forward speed of the piston pig.
The subsea pig launcher described herein addresses the issues identified above. While the subsea pig launcher is described in the following discussion as being installed on a manifold, the present apparatus can be easily adapted for other applications, such as pigging from a subsea wellhead, template or a pipeline end manifold (PLEM) sled at the end of flowline.
More particularly, a method and apparatus have been developed for loading multiple pigs into a subsea pig launcher barrel, storing them inside the barrel over an extended period of time and then releasing the pigs remotely, one at a time, into a subsea flowline or pipeline as needed. The inside diameter of the launcher barrel is preferably slightly larger than the outside diameter of the pigs, so as to allow easy movement of pigs inside the barrel. Since the pigs are pushed inside the barrel by a positive fluid pressure, the system can operate even with the barrel made to fit the pig outside diameter. The top or rearmost pig is constructed so that its outside diameter seals against the inside wall of the barrel, and is therefore referred to as the xe2x80x9cpiston pig.xe2x80x9d
The pigs are advanced inside the barrel by providing hydraulic fluid under pressure behind the piston pig. This advances the stored pigs until the foremost pig is adjacent to a chamber called the xe2x80x9cpig parking chamberxe2x80x9d at the bottom of the barrel. The entrance of the pig parking chamber can be opened to allow the foremost pig to enter the chamber. In the chamber, the foremost pig is held between two pig stops. A kicker line connected to the parking chamber is provided for pumping hydraulic fluid behind the parked pig. By releasing the lower pig stop and pumping hydraulic fluid under pressure through the kicker line, the foremost pig can be launched into another chamber called the xe2x80x9cpig launching chamber.xe2x80x9d The pig launching chamber preferably has an isolation valve at each end, which isolates it from the production header and flowline on one side and the pig parking chamber on the other side. A production kicker line connected to the launching chamber permits introduction of production fluids (hydrocarbons) behind the pig inside the launching chamber. The hydraulic fluid to be used for pushing pigs can be similar to the control fluids conventionally used for subsea systems, or can be any other suitable fluid that has a density greater than seawater, is environmentally acceptable, and is chemically non-reactive with the pig material.
From the launching chamber, the pig is pushed into the production header by opening the isolation valve between the production header and the launching chamber. Once the pig travels from the production header into the flowline, the isolation valve between the launching chamber and the production header is closed. The launching chamber is preferably then flushed with methanol and then, if required, with hydraulic fluid to wash out the residual hydrocarbon fluids.
After all pigs have been launched, the closure at the top of launcher barrel is opened by ROV (remotely operated vehicle) and a new set of pigs is stacked inside the barrel. The apparatus of the present invention, and in particular the pig storage barrel and pig parking chamber, provide several useful advantages over conventional systems. By separating the pig launching from the pig storage, the present system allows the pigs to be stored in hydraulic or control fluid, i.e. fluid that will not degrade the pigs, and also ensures that sufficient fluid volume is available to launch the pigs.
The hydraulic fluid to be used for pushing pigs can be similar to the control fluids conventionally used for subsea systems, or can be any other suitable fluid that has a density greater than seawater, is environmentally acceptable, and is chemically non-reactive with the pig material.